The present invention relates to apparatus for monitoring the operation of the optical repeater of an optical communication system employing the repeater, such that the optical signal transmitted in an optical fiber can be directly amplified when the signal is in the form of light.
The function of monitoring from a terminal station, the operation of optical repeaters made of semiconductor laser amplifiers or optical fiber amplifiers, while continuing the service of an optical communication system, so that an optical signal transmitted in an optical fiber is directly amplified by each repeater at a plurality of stages when the signal is in the form of light, is very important for the stable operation of the system.
FIG. 3 is a block diagram for describing a conventional method of monitoring the operation of an optical repeater. As shown in FIG. 3, the predetermined incoming optical repeater 32 to be monitored is controlled by a prescribed control signal C through an incoming optical fiber 31 from a sending terminal station 30. For the incoming optical repeater 32 appointed by the control signal C, appointed monitoring information is created in a prescribed format by a monitoring circuit 33. An outgoing main line signal S is subjected to prescribed modulation by a created monitoring signal M through an outgoing optical repeater 34 in an outgoing line and then sent to a receiving terminal station 35. For control and modulation of the signals, a method in which an incoming main line signal and the outgoing main line signal are subjected to a low degree of amplitude modulation at a frequency sufficiently lower than the speed of the signals was already proposed by the present applicant in Japanese Patent Application No. 63-270729 entitled "Monitoring and controlling method for optical repeater". When the outgoing main line signal S in subjected to amplitude modulation, the degree of the modulation needs to be kept sufficiently constant so as not to affect the transmission characteristics of the signal. The monitoring information is usually converted into a binary signal which has previously been subjected to modulation so as to become a low-frequency signal by which the outgoing main line signal S is modulated. The signal S is thus subjected to substantially constant amplitude modulation so that the monitoring signal is superposed on the outgoing main line signal which is sent to the receiving terminal station 35. Although conventional amplitude modulation is performed by directly increasing and decreasing the gain of the optical repeater which is comprised of an optical amplifier, the amplification characteristics of the repeater are greatly affected by environmental conditions such as temperature so that it is difficult to keep the degree of modulation constant. For that reason, the degree of modulation of the main line signal is not constant. As a result, the transmission characteristics of the main line signal will be affected.